1. Field of the Invention
The present invention relates to a valve drive mechanism and, more particularly, to a valve drive mechanism for an internal combustion engine.
2. Description of the Related Art
Internal combustion engines often include a valve mechanism for opening and closing an intake valve or exhaust valve. Such a valve mechanism can include a rotating camshaft that is positioned between the respective valves. The camshaft can be rotated by a crankshaft of the internal combustion engine. The camshaft can include one or more cams that rotate with the camshaft. A swing member operates in synchronization with the rotating cam and rocks or swings within a predetermined range. A rocker arm can operate in synchronization with the swing member to open and close the intake valve or the exhaust valve. In certain of these valve mechanisms, in order to reduce the frictional resistance between the swing member and the rocker arm, the rocker arm is provided with a roller. A contact surface is provided on the swing member. The contact surface meets the roller comes into contact on the rocking arm. In this manner, the swing member opens and closes the valves through the rocker arm.
In certain valve mechanisms, when the rocker arm and the respective valves are held in constant contact with each other, the valves can undergo thermal expansion due to a rise in the temperature of the internal combustion engine. This expansion can cause the valve to jump or move upwardly so that each valve presses against the rocker arm towards the swing member. These can cause valve closure action to become unreliable, which can result in gas leakage causing a decrease in engine output. To prevent this upward jumping or movement of the valve, a predetermined valve clearance can be provided between the rocker arm and each valve.
When valve clearance is provided as described above, as the swing member reciprocates and the rocking or swing direction of the swing member is reversed, if there is clearance between the roller and the contact surface as described above, the rotation of the roller on the rocker is retained due to inertia. Thus, at a base circle portion of the swing member, the rocking direction of the swing member and the rotation direction in which the roller rotates becomes opposite to each other. Accordingly, when the roller meets the contact surface of the swing member, wear occurs causing a decrease in durability. In addition, the rocking motion of the swing member may not be accurately transmitted to the rocker arm, which makes it difficult to actuate each valve with reliability.
In particular, when the rotation of the roller is completely retained due to inertia, the relative speed at the time when the roller is separated from the swing member and that at the time when the roller comes into contact with the swing member are substantially same in magnitude but opposite in direction. Thus, the contact surfaces of the two members when in contact exhibit speeds of the same magnitude acting in different directions. According to the elastic hydrodynamic lubrication theory, such a condition is not conductive to the formation of a lubricant film. Thus, it is believed that this condition can easily result in lubricant film breakage causing increased adhesive wear.
In view of the situation described above, a valve mechanism has been designed to prevent adhesive wear between the roller and the contact surface. See, e.g., Japanese Patent Application JP-A-2001-63015. In such a system, the rocker arm is rockably supported by a hydraulic lash adjustor. In this manner, the support position of the rocker arm can be appropriately corrected by the hydraulic lash adjustor to correct the relation between the roller of the rocker arm and the contact surface of the swing member. Accordingly, when the swing member makes reciprocating motion, the swing member and the roller can be always brought into contact with each other. This eliminates a situation where the rocking direction of the swing member and the rotation direction of the roller become opposite to each other, thereby reducing adhesive wear between the contact surface and the roller.